The present invention relates to the field of implanatable medical devices. More particularly, to cardiac pacing systems having atrial capture detection based on far-field R-wave sensing.
Implantable cardiac pacing systems deliver a pacing pulse of sufficient amplitude and width to stimulate the heart. The pacing pulse must be above a threshold value before the heart tissue will respond. Although a pulse much greater than the threshold will evoke a response, a large energy pulse will deplete the implanted battery faster than a pulse just above the threshold. It is desirable to use a pacing pulse that is as small as safely required, one equal to the threshold times a safety factor. The threshold must be measured from time to time, because it changes with such factors as inflammation and fibrous tissue buildup near the electrode of the pacing system, medication, and the patient""s general health. Currently, the threshold test must be performed at a hospital during a time-consuming, manual procedure.
Although much effort has been spent on developing threshold tests that automatically determine the threshold while the cardiac pacing system is in use, automatic atrial threshold tests are still a problem in modern pacemakers. The biggest problem is the measurement of the atrial evoked response, a small signal generated in the atrium immediately after an effective atrial stimulus. This signal is generated in response to the atrial stimulus and is caused by the depolarization of the atrial heart muscle cells. Normally, the atrial evoked response will be masked by the polarization signals. Polarization is a slowly decaying residual signal from the pace pulse and is normally a magnitude (10 to 100 times) bigger than the atrial evoked response, which has a range 0.5 to 5 mV.
There are two basic ways to minimize the polarization artifact. The first is the use of xe2x80x9clow polarizationxe2x80x9d electrodes, electrodes that generate low polarization artifacts due to their construction and the use of specific electrode materials. The second is the use of a triphasic pulse, an output pulse consisting of three phases which can be optimized in such a way that there is no or a very low polarization artifact. The triphasic pulse typically uses two pulses of one polarity separated by a single pulse of the opposite polarity. Although detecting the atrial evoked response is the most direct way of discriminating between a captured and a non-captured output pulse, it is also the most difficult.
In patients with an intact AV conduction system and with electrodes in the atrium and the ventricle, there is another way of performing a threshold test. An effective pulse generated in the atrium will depolarize the atrium and will be passed on to the ventricles via the intact AV conduction system. A sensed ventricular signal (conducted R-wave) is in this case a confirmation of the effectiveness of the atrial stimulus. On the other hand, during a threshold test an ineffective atrial stimulus will not lead to a sensed ventricular R-wave in a specific window after the atrial stimulus. All these systems use the electrode in the ventricle to detect a ventricular depolarization (R-wave) caused initially by an atrial pulse and conducted via an intact AV conduction system. Therefore, these systems require a sensing electrode be installed and functioning in the ventricle.
U.S. Pat. No. 5,954,755 to Casavant discloses a pacing algorithm, and a device and system to implement it that facilitates the task of measuring atrial pacing thresholds and determining atrial capture. The algorithm checks for intact conduction, and in its presence, the measurement is executed in an ADI pacing mode. The test can be used if the patient has no evidence of heart block, enabling the monitoring of ventricular sensed events.
U.S. Pat. No. 5,601,615 to Markowitz et al. discloses a first atrial and ventricular threshold test regimen for use with patients having intact A-V conduction or first degree AV block, A-pace pulses are delivered at a test escape interval and A-V delay. Atrial loss of capture (ALOC) in response to an A-pace test stimulus is declared by the absence of a detected ventricular depolarization (V-event) in the latter portion of the paced A-V delay interval following the delivery of the A-pace test stimulus.
U.S. Pat. No. 5,683,426 to Greenhut et al. discloses an invention pertaining to an apparatus, such as a pacemaker, for detecting a far field R-wave in an atrial lead coupled to the pacemaker. The R-wave may be used for a number of different purposes, depending on the condition of the patient. First, the R-wave may be used to detect the occurrence and progression of AV nodal block in a patient. If AV nodal block is detected, its progression is monitored to determine when such a condition requires additional therapeutic steps. If the clinician confirms that AV nodal block is not present, the R-wave may be used to detect and confirm atrial capture.
Another threshold test system is the double pulse system disclosed in U.S. Pat. No. 5,741,312 to Vonk et al. There is disclosed a pacemaker system with capture verification and threshold testing, in which the pacemaker waits after each change in delivered pace pulses for a stabilization interval, in order to minimize polarization and enhance capture verification. The threshold test utilizes a pace pulse pair, comprising a prior search pulse which is varied during the test, and the regular pacing pulse which is above threshold. The evoked response is detected from the evoked P wave and evoked R wave for atrial capture and ventricle capture, respectively.
The most pertinent prior art patents known at the present time are shown in the following table:
All patents listed in Table 1 above are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, the Detailed Description of the Preferred Embodiments and the Claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the teachings of the present invention.
The present invention is therefore directed to providing a system and method for determining an atrial capture threshold. The system of the present invention overcomes the problems, disadvantages and limitations of the prior art described above, and provides a more efficient and accurate means of determining an atrial capture threshold.
The present invention has certain objects. That is, various embodiments of the present invention provide solutions to one or more problems existing in the prior art respecting the determination of an atrial capture threshold. Those problems include, without limitation: (a) difficulty in measuring the atrial evoked response because the atrial evoked response may be masked by polarization signals, (b) need to measure the atrial evoked response in close proximity to the atrial pacing pulse; (c) infrequent threshold testing, because the threshold test is a time-consuming, manual test that can only be performed in the hospital, (d) need for a ventricular lead using ventricular activity to sense atrial capture, (e) need for special low polarization electrodes to measure atrial evoked response without masking by polarization signals, and (f) need for triphasic test pulses to measure atrial evoked response without masking by polarization signals.
In comparison to known techniques for determining an atrial capture threshold, various embodiments of the present invention provide one or more of the following advantages: (a) the ability to perform frequent, automatic threshold tests; (b) the ability to detect atrial capture by atrial sensing when the polarization signal is relatively large; (c) the ability to detect atrial capture by atrial sensing using the Far Field R-Wave, rather than ventricular sensing; and (d) the ability to perform threshold testing while maintaining regular pacing to the patient.
Some of the embodiments of the present invention include one or more of the following features: (a) an IMD having an automatic threshold test that uses Far Field R-Wave detection to indicate atrial capture, (b) an IMD having an automatic threshold test that uses dual pulses and time shifting of the Far Field R-Wave to indicate atrial capture; (c) methods of performing threshold tests that use Far Field R-Wave detection to indicate atrial capture, (d) methods of performing threshold tests that use dual pulses and time shifting of the Far Field R-Wave to indicate atrial capture, and (e) methods of performing threshold tests that use atrial test pulses and atrial sensing.
At least some embodiments of the present invention involve determining that the patient has a stable Far Field R-Wave (FFRW) pattern that can be used to detect atrial capture, then measuring the FFRW detection time between an atrial pace and FFRW atrial sense. The FFRW detection time is used to determine a FFRW detection window. An atrial pace of a given amplitude and pulse width is applied and the resulting FFRW detected in the FFRW detection window. The atrial test variable, either amplitude or pulse width, is decreased and another atrial pace is applied. If atrial capture is indicated by a FFRW detected in the FFRW detection window, the atrial test variable is decreased and the cycle is repeated. When the atrial test variable has been decreased until no FFRW is detected, that amplitude or pulse width setting is the threshold value and the atrial threshold test ends.
Other embodiments of the present invention involve a double pulse atrial threshold testing method, applying a search pace and a regular pace. It is first determined that the patient has a stable Far Field R-Wave (FFRW) pattern that can be used to detect atrial capture. The FFRW detection time between an atrial pace and FFRW atrial sense is measured. The FFRW detection time is used to determine a regular FFRW detection window where the FFRW normally occurs and a search FFRW detection window before the regular FFRW detection window. The search FFRW detection window precedes the regular FFRW detection window by the time interval between the search pace and the regular pace. A search pace of zero or small amplitude and pulse width is applied and the regular atrial pace applied a time interval later. If no FFRW is detected in the search FFRW detection window, the atrial test variable of the search pace is increased. The atrial test variable can be either amplitude or pulse width, as desired. Another search pace and regular atrial pace are then applied. The cycle is repeated until a FFRW is detected in the search FFRW detection window, indicating atrial capture of the search pace. When the amplitude or pulse width setting of the search pace has been increased until a FFRW is detected in the search FFRW detection window, that amplitude or pulse width setting is the threshold value and the atrial threshold test ends.